Elucidating the pollution characteristics of nitrate, sulfate and ammonium in PM<sub>2.5</sub> in Chengdu, southwest China based on long-term observations

Abstract. Nitrate, sulfate and ammonium (NSA) are the main secondary inorganic aerosols of PM2.5 and play an important role in the process of air pollution. However, few studies have analysed the variation characteristics of NSA in PM2.5 and the effects of control measures through long-term observations. In this study, a long-term observational experiment was conducted from January 1, 2015 to December 31, 2017 in Chengdu, southwest China. NSA pollution characteristics, chemical conversion generation, emission reduction control sensitivity analysis and pollutant regional transport characteristics were analysed. The concentrations of sulfate and ammonium in PM2.5 have been well reduced, but the effect of reducing nitrate was not obvious. Seasonal and diurnal variations have obvious characteristics, winter still has a high NSA concentration and emission intensity, and the concentration during the day was higher than that at night. Although the workday concentration was slightly higher than the weekend concentration, the difference was nonsignificant. The chemical conversion characteristics of NSA formation were comprehensively analysed, and the aqueous phase oxidation process plays an important role in the conversion of NOx, SO2 and NH3 to NSA. The ammonia-rich environment became increasingly obvious in the atmosphere of Chengdu. Under these conditions, the sensitivity of NSA concentration variation was analyses using the ISORROPIA-II thermodynamic model, and the results show that by reducing NOx and SO2 emissions, not only can reduce the nitrate and sulfate in PM2.5, but also help reduce the formation of ammonium nitrate and ammonium sulfate to reduce ammonium. The results also show that while carrying out NSA emission reduction, it is also possible to generate potential risks of changes in aerosol pH. Combined with meteorological conditions and a potential source contribution function (PSCF) analysis, local emissions and regional emissions of pollutants are found to have important impacts on Chengdu's atmospheric environment. This research result not only provides an assessment of the current atmospheric emission reduction effect but also provides an important reference for determining methods to further reduce the NSA concentration in atmospheric PM2.5.